Field of the Invention
The present invention relates generally to methods for the monitoring of nerves and specifically to the detection of reversible nerve injury.
Description of the Related Art
Reversible Nerve Injury, known as “neuropraxia,” is a medical condition where a motor nerve is stressed, often occurring during surgery, by surgical trauma, stretching, and/or devascularization. Because neuropraxia is preventable, it is of value to identify degrees of neuropraxia early and to quantify the extent of nerve injury. An insult capable of causing neuropraxia may be capable of resulting in irreversible nerve palsy. This is an additional concern and reason for detecting and preventing neuropraxia.
Historically, neuropraxia prevention was attempted by Intraoperative Nerve Monitoring (IONM). This technique involved collection of electromyographic responses (EMG) from the target muscle innervated by target nerves and “monitoring” those responses for waveform patterns typical of irritation of the nerve. IONM response might include sustained EMG activity due to stretch or tension on the nerve from retraction. IONM EMG response of significance might also include bursts of activity associated with a dissection recognized as being risky by the surgeon (surgeon identified response) or activity which is seen by an individual monitoring the IONM equipment, such as visual or auditory feedback from the EMG.
A relatively new modality of neuropraxia detection is the observation of evoked responses of the nerve. In this method, a nerve response is “evoked” by an electronic nerve stimulator and the response is examined for indications of the functional integrity of the stimulated nerve. Indications might include latency of response (the time from the stimulation to the onset of the evoked response), the morphology of the response waveform, and the amplitude of the evoked response voltage. This type of testing can be automatic by “ping” testing with an implantable stimulator probe, or manually by the surgeon using a hand held stimulation probe. FIG. 1 depicts an example of a waveform 100 of an evoked EMG response 102 wherein the response plotted on a graph wherein the y-axis 104 represents electric response output and the x-axis 106 represents time. Prior art methods observe changes in the amplitude (peak-to-peak) values 108 or differentiation (i.e. the slope of the waveform) to infer the onset of neuropraxia. However, this is not a completely accurate representation of the strength of a nerve response or the integrity of the monitored nerve.
Present methods of Neuropraxia detection, especially those requiring implantation of dedicated stimulation devices and those relating to voltage response algorithms may be costly, time consuming, suffer from lack of reliability, and counter-intuitive in conceptualization, possibly leading to misdiagnosis during the stress of performing live surgery. In addition, voltage responses do not always truly reflect the degree of injury to a nerve, the response also being dependent on the strength of stimulation, the size and excitability of the target muscle, and the effectiveness of the pickup electrodes. Of particular disadvantage is the fact that nerve response can occur without actual muscle movement, giving a false negative indication of advancing neuropraxia. A more reliable and efficient method for monitoring the heath and response strength of a nerve is therefore needed.